A conventional ultrasonic probe comprises a transducer package which must be supported within the probe housing. As shown in FIG. 1, a conventional transducer package 2 comprises a linear array 4 of narrow transducer elements. Each transducer element is made of piezoelectric material. The piezoelectric material is typically lead zirconate titanate (PZT), polyvinylidene difluoride, or PZT ceramic/polymer composite.
The design and fabrication of individual transducer elements with desirable acoustic properties, e.g., high sensitivity, wide bandwidth, short impulse response, and wide field of view, is a well known art.
Typically, each transducer element has a metallic coating on opposing front and back faces to serve as electrodes. The metallic coating on the front face serves as the ground electrode. The ground electrodes of the transducer elements are all connected to a common ground. The metallic coating on the back face serves as the signal electrode. The signal electrodes of the transducer elements are connected to respective electrical conductors formed on a flexible printed circuit board (PCB) 6.
During operation, the signal and ground electrodes of the piezoelectric transducer elements are connected to an electrical source having an impedance Z.sub.s. When a voltage waveform is developed across the electrodes, the material of the piezoelectric element contracts at a frequency corresponding to that of the half-wave resonance of the piezoelectric thickness, thereby emitting an ultrasonic wave into the media to which the piezoelectric element is coupled. Conversely, when an ultrasonic wave impinges on the material of the piezoelectric element, the latter produces a corresponding voltage across its terminals and the associated electrical load component of the electrical source.
In conventional applications, each transducer element produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter (not shown). The pulses are transmitted to the transducer elements via the flexible PCB 6. This ultrasonic energy is transmitted by the probe into the tissue of the object under study. The ultrasonic energy reflected back to transducer element array 4 from the object under study is converted to an electrical signal by each receiving transducer element and applied separately to a receiver (not shown).
The transducer package 2 also comprises a mass of suitable acoustical damping material having high acoustic losses, e.g., silver epoxy, positioned at the back surface of the transducer element array 4. This backing layer 12 is coupled to the rear surface of the transducer elements to absorb ultrasonic waves that emerge from the back side of each element so that they will not be partially reflected and interfere with the ultrasonic waves propagating in the forward direction.
Typically, the front surface of each transducer element of array 4 is covered with a first acoustic impedance matching layer 8 shown in FIG. 1. The first matching layer 8 may consist of a glass material such as Pyrex.RTM. borosilicate glass. Typically, a second acoustic impedance matching layer is later bonded to the first acoustic impedance matching layer. The impedance matching layers transform the high acoustic impedance of the transducer elements to the low acoustic impedance of the human body and water, thereby improving the coupling with the medium in which the emitted ultrasonic waves will propagate.
The transducer element array, backing layer and first acoustic impedance matching layer are all bonded together in a stack-up arrangement, as seen in FIG. 1.. During assembly of the ultrasonic probe, the transducer stack-up must be held securely within the probe housing (not shown in FIG. 1). Typically, this is accomplished by securing the transducer stack-up within a four-sided array case 14, i.e., a "box" having four side walls but no top or bottom walls, as shown in FIG. 2. The array case is made of electrically conductive material and provides a common ground for connection with the ground electrodes of the transducer elements. Preferably the array case is made of graphite.
The transducer stack-up is inserted into a recess in the array case 14 until the bottom surface of the first acoustic impedance matching layer 8 is flush with the bottom edge of the array case. The transducer stack-up is conventionally bonded inside the array case using epoxy. Then a second acoustic impedance matching layer 10 is conventionally bonded to those flush bottom surfaces (see FIG. 2). Matching layer 10 may consist of a plastic material, such as Plexiglas.RTM. acrylic resin plastic.
During assembly of an ultrasonic probe incorporating the structure of FIG. 2, transducer package 2 must be secured within the probe housing (not shown). The interior volume of the probe housing surrounding the transducer package is filled with thermally conductive potting material, e.g., heat-conductive ceramic granules embedded in epoxy. The potting material stabilizes the construction and assists in dissipating heat, generated during pulsation of the transducer element array, away from the probe surface/transducer face toward the interior/rear of the probe.
In the transmission mode, the piezoelectric ceramic of the transducer elements alternately compresses and expands in response to electrical signals received from a pulser circuit (not shown) via coaxial cables (not shown) electrically connected to the flexible PCB 6. The resulting compression waves propagate in both the forward and rearward directions, with the rearward-propagating compression waves being damped by the backing layer 12.
In the receiving mode, the piezoelectric ceramic of the transducer elements alternately compresses and expands in response to compression waves reflected back to the probe by the object being ultrasonically examined. These waves are transduced into electrical signals which are carried to a receiver circuit (not shown) by the flexible PCB 6 and the coaxial cable connected thereto.
The coaxial cables typically have a resistance of 50 to 100 ohms. For ideal operating conditions, it is desirable that the transducer elements have an electrical resistance equal to that of the coaxial cable to which they are electrically coupled. However, conventional transducer elements typically have a resistance on the order of 200 ohms, i.e., a resistance greater than the optimal resistance by a factor of 2 to 4.